Maleic anhydride may be used as a raw material in the production of many products, such as synthetic resins, and may generally be prepared by the catalytic oxidation of n-butane. The catalyst of choice for this oxidation is typically a catalyst containing vanadium, phosphorus, oxygen (VPO) and optionally a promoter component.
These catalysts are generally prepared by contacting vanadium-containing compounds with phosphorus-containing compounds and optionally promoter component containing-compounds under conditions suitable to reduce the pentavalent vanadium to the tetravalent state to thereby form a catalyst precursor containing vanadyl hydrogen phosphate and optionally the promoter component. The catalyst precursor may then be recovered and typically formed into a shaped body, such as a tablet or pellet, by compression in a die. A lubricant is ordinarily incorporated as well to aid in the tableting or pelleting process. The pellet or tablet may then be subjected to calcination to transform the catalyst precursor into an active catalyst.
Variants and different embodiments of the preparation of the active catalyst are further described in, for example: U.S. Pat. No. 4,567,158, which discloses preparation of the catalyst precursor in the presence of an alcohol-modifying agent to form a highly porous catalyst precursor which is then converted to the active catalyst; U.S. Pat. No. 4,996,179 which discloses forming the catalyst precursor into a geometric shape and calcining the shaped catalyst in an inert atmosphere at a temperature within the range of about 343° C. to 704° C. and further at an increased temperature in an oxygenous atmosphere to produce the active catalyst; U.S. Pat. No. 5,137,860 which discloses the use of three heating stages for converting the catalyst precursor into an active catalyst; U.S. Pat. No. 5,506,187 which discloses preparation of the catalyst precursor in the presence of a glycol ether solvent; U.S. Pat. No. 5,530,144 which discloses the use of orthophosphoric acid as the phosphorus-containing compound for producing the catalyst precursor; and U.S. Pat. No. 5,773,382 which discloses the use of removable pore modification agents in the preparation of the catalyst precursor to produce a catalyst precursor exhibiting a high proportion of large pores.
Thus, there are many different methods of producing and activating these catalysts, however the main active substance in all such catalysts is believed to be vanadyl pyrophosphate (VO)2P2O7. This active substance is believed to be very sensitive to preparation conditions in terms of catalytic performance. In the past, improvements in catalytic performance have come from processing techniques including (1) dopant addition, such as Fe, Mo, Nb, Zr as promoters, for example, as described in U.S. Pat. No. 5,158,923; and (2) catalyst structure modification including catalyst shape and pore structure inside of catalyst particles, for example, as described in U.S. Pat. No. 5,168,090. Although these and other known techniques provide catalysts having appreciable activity and selectivity in the preparation of maleic anhydride, further improvements are desirable.